1. Field of the Invention
The present invention relates to a surface modifier for a transparent oxide electrode, to a surface-modified transparent oxide electrode, and to a method for producing a surface-modified transparent oxide electrode. The present invention particularly relates to a surface modifier for a transparent oxide electrode that has a greatly advantageous effect in increasing the work function of the transparent oxide electrode and provides good wettability of the modified electrode surface to organic solvents, a surface-modified transparent oxide electrode, and a method for producing a surface-modified transparent oxide electrode.
2. Description of the Related Art
Transparent conductive films, which have high visible light transmittance, are used as transparent electrodes for display devices such as liquid-crystal displays, organic EL displays, and electronic paper; and for touch panels, organic solar cells, and the like. Use of organic conductive polymers, carbon nanotubes, graphenes, and the like as materials for transparent conductive films are considered, in addition to metal thin films. Compounds called transparent conductive oxides, which absorb a small amount of visible light and exhibit high electric conductivity among inorganic metal oxides, are now mainly used.
As specific transparent conductive oxides, tin oxides, zinc oxides, indium oxides, and titanium oxides are known. Among these, tin-doped indium oxide (hereinafter, also referred to as “ITO”), which is indium oxide doped with tin, is a widely used material for reasons such as its ability to be made into thin films due to its low volume resistivity, its ability to increase transmittance, and its ease of being patterned. Transparent electrodes made of transparent conductive oxides are herein called transparent oxide electrodes.
In the case in which ITO is used in organic EL devices, ITO is often used as anodes because the work function of ITO itself is relatively high. One of important technical challenges of organic EL devices is to reduce the drive voltages. As such, a hole injection barrier from an anode to a hole transport layer must be reduced. Hole transport materials have an ionization potential from about 5.2 to 5.8 eV, and the difference with the work function of ITO will be the hole injection barrier. Since the work function of ITO is around 4.5 to 4.8 eV, there exists a high injection barrier of about 0.4 to 1.3 eV. Several methods for reducing this injection barrier have been proposed.
Generally, a method for inserting a hole injection layer having an ionization potential intermediate between the work function of the anode and the hole transport layer material has been adopted. As the material for the hole injection layer, various materials such as phthalocyanines, porphyrins, triarylamines, polyethylenedioxythiophene/polystyrene sulfonate (PEDOT/PSS), and transition metal oxides are used.
In recent years, it has been reported that forming a monolayer on an anode surface by using predetermined compounds increases hole injectionability. This monolayer formation replaces the above-described hole injection layer and can also eliminate the hole injection layer, allowing devices to be thinner. For example, Non-Patent Literature 1 (Japanese Journal of Applied Physics, 2008, vol. 47, pp. 455-459) discloses that depositing a self-assembled monolayer (F-SAM) of heptadecafluorodecyl triethoxysilane on ITO enhances the performance of organic EL devices compared to the case in which copper phthalocyanine is used as a hole injection layer. As the factor, it is indicated that the stability of N,N′-di-1-naphthyl-N,N′-diphenylbenzidine (NPD) layer is enhanced compared to ITO having no F-SAM by increase in the work function of the ITO surface due to formation of an F-SAM and deposition of NPD, which is a hole transport layer, on the ITO with an F-SAM.
A method using a silane derivative is excellent in that a layer can be formed in a short time. Additionally, since silane derivatives have a relatively high vapor pressure, layer deposition in a gas phase is possible, in addition to layer deposition in a liquid phase. Furthermore, it is also preferable that layers with fewer impurities can be deposited. However, an ITO surface with an F-SAM has very high water and oil repellency due to its low surface free energy. As the result, contact angles of solvents and solutions are increased. Patent Literature 1 (Japanese Patent Laid-Open No. 2008-130882) discloses a method for making silicon thermal oxidation layers highly liquid-repellent by a surface treatment using various silanes having fluorinated hydrocarbon groups.
In contrast, Non-Patent Literature 2 (Thin Solid Films, 2001, vol. 394, p. 292-297) and Non-Patent Literature 3 (Journal of Materials Chemistry, 2002, vol. 12, pp. 3494-3498) discloses that surface modification of ITO by use of a silane derivative having a particular structure increases the work function of ITO. As described above, the purpose of increasing the work function of ITO is, for example in the case of organic EL devices, is to adapt the ionization potential of the anode to the ionization potential of the hole transport layer to reduce the hole injection barrier. While the hole injection barrier from non-surface modified ITO to a hole transport layer is generally 1 eV or more, the amount of change in the work function of ITO is about +0.3 eV in Non-Patent Literature 2 and +0.5 eV in Non-Patent Literature 3. The amount of change in these work functions is still insufficient.